This invention relates to a method of performing ion implantation on a silicon substrate, and more particularly, it relates to an ion implantation method by means of which planar channeling in the substrate can be reduced.
Ion implantation is widely used in the manufacture of semiconductors in order to introduce impurities into silicon substrates on account of the superior reproducibility, uniformity, and controllability which can be achieved. FIG. 1 schematically illustrates a conventional electrostatic scanning ion implantation apparatus by means of which ion implantation is commonly carried out. As shown in this figure, ions are emitted from an ion source 1. Of these ions, undesired ones are removed by an analysis magnet 2. An ion beam 6 containing the desired dopant ions exits from the analysis magnet 2 and passes through Y-scan electrode plates 3 and X-scan elecrode plates 4, which control the direction of the ion beam 6. The ion beam 6 is directed at a silicon wafer 7 which is supported by a base 5. In order to reduce axial channeling of the dopant ions, the wafer 7 is usually tilted by an angle T so that the angle of incidence of the ion beam 6 with respect to a normal to the surface of the wafer 7 is about 7.degree..
FIG. 2 is a sheet resistance map of the surface of a silicon wafer 7 which was subjected to ion implantation by a conventional method using the apparatus of FIG. 1. Sheet resistance was determined by the four-probe method of resistivity measurement. The wafer 7 was made of (100) Si having a wafer flat 7a lying in a (110) crystal plane. The angle of tilt was 7.degree., and the angle of the wafer flat 7a with respect to the horizontal was 0.degree.. Implantation of boron ions was performed at 50 keV at a dose of 2.times.10.sup.13 atoms per square cm, after which annealing was performed at 900.degree. C.
The two bold contour lines in FIG. 2 are reference values, and the other contour lines are drawn at intervals of 0.5% of the reference values. The (+) marks indicate regions where the sheet resistance was larger than the reference value, and the (-) marks indicate regions where the sheet resistance was smaller than the reference values.
As is clear from FIG. 2, there is a belt-shaped region of low sheet resistance at the center of the wafer 7, and the sheet resistance increases towards the top and bottom of the wafer 7. The cause of this decreased sheet resistance is not an increased dose of dopants at the center of the wafer 7. Rather, it is due to planar channeling. Namely, even though axial channeling along the &lt;100&gt; crystal axes was prevented by tilting the substrate by 7.degree. with respect to the ion beam 6, as the wafer flat 7a was parallel to the horizontal, planar channeling took place between the (110) crystal planes, and dopant ions penetrated deeper into the surface of the wafer 7 than desired, just as when axial channeling occurs. When using an electrostatic scanning ion implantation apparatus in which an ion beam is scanned in the X and Y directions, the ion beam can easily become aligned with the (110) crystal planes at the center of the wafer 7, so that in the center of the wafer 7, the depth of implantation increases and the sheet resistance decreases.